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DOI: 10.7343/as-2016-224
Special Issue AQUA2015 - Paper
A proposal of conceptual model for Pertuso Spring discharge evaluation
in the Upper Valley of Aniene River
Proposta di modello concettuale per la stima della portata della sorgente Pertuso nell’Alta
Valle del fiume Aniene
Giuseppe Sappa, Flavia Ferranti, Francesco Maria De Filippi
Riassunto: Il bacino idrogeologico della parte alta del Fiume
Aniene appartiene ad un grande acquifero carsico, che interagisce con il fiume e rappresenta la più importante risorsa idrica
nella parte sud-orientale della Regione Lazio (Italia Centrale)
usata per approvvigionamento potabile, agricolo e idroelettrico.
Il presente lavoro fornisce i dati idrogeochimici e alcune loro
interpretazioni per una sorgente e 2 sezioni di misura del Fiume Aniene, monitorate da luglio 2014 a dicembre 2015 nell’Alta
Valle del Fiume Aniene. Lo scopo principale è stato quello di
definire le vie di deflusso delle acque sotterranee e i processi idrogeochimici che governano le interazioni tra acque sotterranee e
superficiali in questa zona. Questo articolo è perciò il risultato di attività eseguite per il piano di monitoraggio ambientale
nell’ambito del progetto di captazione della Sorgente Pertuso,
che sarà sfruttata per rifornire un’importante rete idrica nella
zona Sud di Roma. Sono state analizzate le misure di portata e i
dati idrogeochimici al fine di sviluppare un modello concettuale
di interazione tra fiume e acquifero, con l’obiettivo di raggiungere un corretto grado di gestione e protezione di questo importante sistema idrogeologico. Tutti i campioni d’acqua sono del tipo
Ca-HCO3 (bicarbonato-calcica). La modellazione geochimica e il
calcolo dell’indice di saturazione per i campioni di acqua mostrano che la chimica delle acque sotterranee e superficiali nell’area
di studio risente dell’interazione con i minerali carbonatici. Tutti i campioni di acqua sotterranea risultano sottosaturi rispetto
alla calcite e alla dolomite, tuttavia alcuni campioni del Fiume
Aniene sono saturi rispetto alla dolomite. L’analisi dei rapporti
Mg2+/Ca2+ indica che la dissoluzione dei minerali carbonatici
gioca un ruolo significativo nella chimica delle acque sotterranee
e superficiali, dipendendo dai processi idrogeologici che controllano il tempo di residenza delle acque sotterranee e gli equilibri
chimici nell’acquifero.
Parole chiave: acquifero, carsismo, modellazione idrogeochimica,
Abstract: The Upper Aniene River basin is part of a large karst
aquifer, which interacts with the river, and represents the most important water resource in the southeast part of Latium Region, Central
Italy, used for drinking, agriculture and hydroelectric supplies. This
work provides hydrogeochemical data and their interpretations for 1
spring and 2 cross section of Aniene River, monitored from July 2014 to
December 2015, in the Upper Valley of Aniene River, to identify flow
paths and hydrogeochemical processes governing groundwater-surface
water interactions in this region. These activities deal with the Environmental Monitoring Plan made for the catchment work project of the
Pertuso Spring, in the Upper Valley of Aniene River, which is going to
be exploited to supply an important drinking water network in the South
part of Rome district. Discharge measurements and hydrogeochemical
data were analyzed to develop a conceptual model of aquifer-river interaction, with the aim of achieving proper management and protection
of this important hydrogeological system. All groundwater samples are
characterized as Ca-HCO3 type. Geochemical modeling and saturation index computation of the water samples show that groundwater
and surface water chemistry in the study area was evolved through the
interaction with carbonate minerals. All groundwater samples were
undersaturated with respect to calcite and dolomite, however some of
the Aniene River samples were saturated with respect to dolomite. The
analysis of Mg2+/Ca2+ ratios indicates that the dissolution of carbonate
minerals is important for groundwater and surface water chemistry,
depending on the hydrological processes, which control the groundwater
residence time and chemical equilibria in the aquifer.
monitoraggio, tracciante ambientale.
Keywords: : aquifer, karst, hydrogeochemical modelling, monitoring,
environmental tracer.
Giuseppe Sappa, 
Department of Civil, Building and Environmental Engineering
Sapienza University of Rome
via Eudossiana 18, 00184, Rome, Italy
Tel: +39.06.44585010 - Fax: +39.06.233239345
[email protected]
Flavia Ferranti, Francesco Maria De Filippi
Department of Civil, Building and Environmental Engineering
Sapienza University of Rome
via Eudossiana 18, 00184, Rome, Italy
Ricevuto: 24 giugno 2016 / Accettato: 14 settembre 2016
Pubblicato online: 3 ottobre 2016
This is an open access article under the CC BY-NC-ND license:
http://creativecommons.org/licenses/by-nc-nd/4.0/
© Associazione Acque Sotterranee 2016
Acque Sotterranee - Italian Journal of Groundwater (2016) - AS18- 224: 49 - 57
49
Special Issue AQUA2015 - DOI: 10.7343/as-2016-224
Introduction
Most of groundwater in the southeast part of Latium
Region, as in the whole Apennine Mountains chain, is stored
in karst aquifers. Karst groundwater depletion is mostly due
to the increasing of anthropogenic activities and the impacts
of climate change (Guo et al. 2005, Sappa et al. 2013).
Thus, groundwater exploitation in karst aquifers requires
special management strategies to prevent their quality and
quantity depletion and to support decision-making for
water resources management (Sappa et al. 2013, Foster et
al. 2013). In karst aquifers, the fast underground outflow in
the saturated zone (White 1969, White 2002) is due to the
heterogeneous distribution of permeability (Zwahlen 2004,
Bakalowicz 2005), because there are many conduits and voids
developed by the dissolution of carbonate minerals. Therefore,
quantification of karst spring discharge and water budget
formulation requires defining the karst network geometry,
which is not, every time, measurable with reliability
(Cherubini et al. 2008).
For this reason, environmental tracers are useful tools
to provide information about groundwater flowpaths and
residence times in karst aquifers without allowing for the
specification of input locations and times (Clark and Fritz
1997, Mazor 2004).
Environmental tracers provide information about the
flow and mixing processes of water coming from different
sources (Guida et al. 2013). They are also useful to point
out directions of groundwater flow and to determine origin
and residence times of karst groundwater (Batiot et al. 2003,
Sappa et al. 2015a, 2015b). Typical natural tracers are major
ions, trace elements, dissolved organic carbon and water
isotopes (Hunkeler and Mudry 2007, Leibundgut et al. 2009).
Groundwater circulating in karst aquifer generally has great
concentrations of Calcium and Magnesium as the result of
bedrock weathering processes. Variations of Ca2+ and Mg2+
in groundwater are used very successfully as natural tracers in
studies aiming to evaluate groundwater residence time within
the karst aquifers (Batiot et al. 2003). The changes in Ca2+ and
Mg2+ concentration values mainly depend on the residence
time of groundwater in karst systems, which are controlled
by the volume and mechanism of recharge, the distance from
the recharge area and the dissolution of carbonate minerals
(Langmuir 1971, White 1988). The dissolution kinetics of
Magnesium is longer than that of Calcium, so the increasing
Mg2+/Ca2+ ratio implies the saturation of water with calcite,
highlighting long residence time and enhanced weathering
along the groundwater flow paths (White 1988, Edmunds et
al. 1987). The Mg2+ content in groundwater also depends on
other parameters such as chemical and mineralogical purity
of limestone and presence of dolomite within the rock masses
they flow across (Mudarra and Andreo 2011). Nevertheless,
the Mg2+ concentration values increase and hence
Mg2+/Ca2+ ratio depend not only on the dissolution/
precipitation reaction of calcite and dolomite. In fact, a faster
dissolution of dolomite may be due to an increasing in water
temperature (Herman and White 1985). Thus, environmental
tracers and hydrochemical investigation techniques provide
much information about groundwater flow systems and the
main hydrogeochemical processes affecting the composition
of groundwater within the karst aquifers (Tooth and Fairchild
2003) and their interaction with surface water.
Geological and hydrogeological setting
Karst aquifer feeding Pertuso Spring is located in the Upper
Valley of the Aniene River (Latium, Central Italy), along the
SW boundary of the Simbruini Mountains, in the outcrop of
Triassic-Cenozoic carbonate rocks, locally covered by fluvial
and alluvial deposits and Quaternary sediments (Ventriglia
1990). The stratigraphic succession of dolomite, dolomitic
limestone and limestone is distributed homogeneously in the
Valley (Damiani 1990). Dolomite is the dominant lithofacies,
characterized by white and gray crystalline dolomite, with
some breccia levels. Over this geological formation, limestones
and dolomites of Upper Cretaceous age are present, and their
immersion is concordant with the Triassic dolomite (Damiani
1990). Karst springs are numerous along the first part of the
Fig. 1 - Simpliﬁed geological map of Pertuso Spring hydrogeological
basin and location of Aniene River gauging sections.
Fig. 1 - Carta geologica semplificata del bacino idrogeologico
della sorgente Pertuso e posizione delle stazioni di misura lungo
il fiume Aniene.
50
Aniene River and rise in the Triassic dolomitic formations
(Fig.1).
This karst system is characterized by one main outlet,
Pertuso Spring, and several springs which inflow into the
Aniene River, at different points (Tab.1).
Due to their lower solubility and their ductility, dolomites
are less fractured than limestone. As a consequence of it in the
Aniene River Basin, almost all groundwater come out, where
there is a limestone-dolomite contact (Ventriglia 1990) (Fig.1)
and this latter one is below the former one.
Pertuso Spring (elevation of 698 m a.s.l.) is located between
Filettino and Trevi nel Lazio (FR) and belongs to the Special
Area of Conservation (SAC) of Aniene River Springs,
established under Directive 92/43/EEC. This spring is the
main outlet of this karst aquifer and comes out of alluvial
sediments, covering a very thick layer made of Cretaceous
limestone (Cipollari et al. 1995). The spring water is partially
caught for hydroelectric and drinking supplies, whereas the
remaining part directly flows into the Aniene River. The
Pertuso Spring hydrogeological basin collects groundwater
coming from a 50 km2 catchment area (Sappa and Ferranti
2014) bounded by the Triassic dolomite to the NE (Fig.1).
This aquifer is made, for the most part, of Cretaceous karst
limestone. The base of the stratigraphic series is made of
Upper Cretaceous carbonates, represented by the alternation
of granular limestone and dolomites layers. Above these
ones Quaternary fluvial and alluvial deposits lie, downward
pudding and Miocene clay and shale (Ventriglia 1990).
Rainfall is the primary source of recharge to this karst aquifer,
feeded fastly through karst features such as sinkhole, dolines,
swallow holes and fractures (Bono and Percopo 1996).
Pertuso Spring reacts very fastly to precipitation events,
with significant increases in discharge rates, which are
proportional to the intensity of rainfalls. Groundwater coming
from Pertuso Spring is collected within an aquifer mostly
made by a well-known volume of carbonate rocks (White
2002). The most distinctive feature of Pertuso karst spring
is the branching network of conduits that increases in size in
the downstream direction (Fig.2). The largest active conduit
drains the groundwater flow coming from the surrounding
aquifer matrix, the adjoining fractures and the smaller nearby
conduits (White and White 1989). This conduit network is
able to rapidly discharge large quantities of water through
this karst aquifer (up to 3 m3/s).
Tab. 1 - Main karst springs of Upper Valley of Aniene River (ACEA ATO 2 S.p.A.,
unpublished data, 2005).
Tab. 1 - Principali sorgenti carsiche dell’Alta Valle dell’Aniene (ACEA ATO 2
S.p.A., dati non pubblicati, 2005).
Spring
Altitude (m a.s.l.)
Average discharge (L/s)
Acqua Santa
900
65
Acqua Nera
1030
80
Fonte del Forno
950
164
Cesa degli Angeli
940
200
Radica
1110
250
Pertuso
698
1400
Fig. 2 - Plan view of the development of Pertuso Spring karst drainage network (modified from ACEA ATO 2 S.p.A., unpublished data, 2005).
Fig. 2 - Sviluppo planimetrico della rete carsica della Sorgente Pertuso (modificata da ACEA ATO 2 S.p.A., dati non pubblicati, 2005).
Materials and Methods
This paper presents part of the results of more than one year
of monitoring of the Environmental Monitoring Plan, related
to the catchment project of the Pertuso Spring. Water samples
have been collected from three sampling points within the
study area, i.e. one from Pertuso Spring, the others from two
gauging stations located along the Aniene River, respectively,
upstream (SW_01) and downstream (SW_02) the spring
(Fig.1). These gauging stations belong to the monitoring
network set up for the Environmental Monitoring Plan in
agreement with the Ministerial Decree 260/2010, chosen to
focus on the sensitive connections between surface water and
groundwater (Sappa and Ferranti 2014). According to the
Environmental Monitoring Plan, groundwater and surface
water have been monitored seasonally (4/year) with the aim
of set up the hydrogeological conceptual model of the karst
aquifer (Sappa and Ferranti 2014).
The experimental data, referred to the period July 2014
- December 2015, were obtained from field investigations
and from chemical laboratory analyses. Water temperature,
electrical conductivity and pH values were determined in
field using HANNA HI9813-6 waterproof hand-held meter
(Hanna Instruments, Woonsocket, RI, USA). Geochemical
analyses were carried out at the Geochemical Laboratory of
Sapienza University of Rome. Water samples were filtered
through cellulose filters (0.45 μm), and their major and
minor constituents were determined by ion chromatography
(IC) by a 761 professional IC Metrohm (reliability ±2%)
(Metrohm, Herisau, Switzerland). Bicarbonate (HCO3-) was
determined by titration with 0.1 N HCl (reliability ±2%). A
statistical summary of the major physicochemical parameters
is shown in Tab.2.
The geochemical modeling program PHREEQC, version
3.0 (Parkhurst and Appello 1999), implemented with the
51
Tab. 2 - Summary statistics for in situ measurements of physicochemical parameters and chemical concentrations of constituents of water samples (ions in mg/L, electrical conductivity in μS/cm, T in °C, hardness in °F).
Tab. 2 - . Statistica riassuntiva dei parametri fisico-chimici misurati in situ e delle concentrazioni chimiche dei campioni d’acqua (ioni in mg/L, conduttanza
elettrica in mS/cm, T in °C, durezza in °F).
Parameters
Min
SW_01
Max
Mean
Min
Pertuso spring
Max
Mean
Min
SW_02
Max
Mean
Ca2+
53.4
57.8
55.0
48.9
53.3
50.8
51.3
55.6
52.7
Mg2+
23.6
25.2
24.4
8.3
10.4
9.5
12.0
14.7
12.9
Na+
2.3
2.6
2.4
1.8
2.0
1.9
2.0
2.3
2.1
K+
0.4
0.5
0.4
0.3
0.5
0.4
0.3
0.7
0.43
281.8
302.6
290.6
206.0
238.0
218.8
218.4
235.5
225.7
2.9
3.2
3.0
2.3
2.5
2.4
2.5
2.7
2.6
3.8
4.5
4.2
3.3
3.7
3.5
3.3
4.2
3.7
HCO3SO4
2-
ClNO3
-
T
0.8
1.3
1.1
0.9
1.2
1.0
0.9
1.1
1.0
5.8
11.3
8.56
8.0
9.5
8.5
6.7
10.7
8.3
pH
8.0
8.5
8.4
7.8
8.0
7.9
8.1
8.2
8.2
EC
356.0
395.0
384.4
283.0
291.0
288.0
294.0
317.0
310.2
Hardness
23.1
24.8
23.8
16.1
17.4
16.6
17.9
19.3
18.5
thermodynamic dataset wateq4f.dat, was employed to assess
the state of equilibrium among groundwater, surface water
and carbonate minerals present in terms of saturation index.
The saturation index (SI) indicates the chemical equilibrium
potential between water and minerals and the tendency for
water-rock interaction (Wen et al. 2008). Negative values
of SI highlight that water samples are undersaturated with
respect to a particular mineral, indicating the possibility
of mineral phase dissolution by groundwater and, thus, a
potential source of constituents. Likewise, SI>0 reflects the
oversaturated state, showing the possibility of mineral phase
precipitation, thus limiting the constituent concentration.
SI=0 represents the mineral phase equilibrium state. The
discharge measurements were carried out along the Aniene
River, upstream (SW_01) and downstream (SW_02) Pertuso
Spring, by the application of traditional current meter.
According to the U.S. Geological Survey (USGS) procedure,
stream discharge has been calculated as the product of the
cross section area by the average stream flow velocity in the
cross section obtained using a current meter (US EPA Region
6 2003, BS EN ISO 2007). The main equipment needed to
measure the stream flow velocity is a SEBA horizontal axis
current meter F1 (SEBA Hydrometrie GmbH & Co. KG,
Kaufbeuren, Germany), having a propeller diameter of 80
mm which, combined with SEBA Z6 pulse counter, allows to
measure velocity between 0.025 m/s and 10 m/s. The SEBA
current meter has been used according to EN ISO 748:2007
requirements (BS EN ISO 2007). This current meter method
gives the local water velocity in each vertical following the
application of a calibration equation between stream velocity,
v (cm/s) and the number of spins, n (s-1) (1).
n=0.82+33.32n
(1)
in this study. Ca2+, Mg2+ and HCO3- represent more than
80% of the dissolved solids in water samples and these
concentrations are influenced by the dissolution of carbonate
minerals, forming limestone, which are the most dominant
formations outcropping in the study area. The hydrochemical
facies of groundwater and surface water were studied by
plotting the concentrations of major cations and anions in the
Piper trilinear diagram (Fig.3) (Piper 1944).
Fig. 3 - Piper plot for hydrochemical facies classification of groundwater and surface
water.
Fig. 3 - Diagramma di Piper per la classificazione in facies idrochimiche delle
acque sotterranee e superficiali.
Results and Discussion
In order to establish the groundwater discharge pattern,
hydrochemical data and discharge measurements were used
Based on the dominance of major cationic and anionic
species two hydrochemical facies have been identified: i) CaMg-HCO3 and ii) Ca-HCO3. These results are due to the
presence of limestone, dolomitic limestones and dolomites
outcropping in the study area. Water samples coming from
SW_01 gauging station present higher values of concentration
in Mg2+, while those coming from Pertuso Spring are
definitely poorer in it, and show a clear composition of CaHCO3 water type (Fig.3). The different hydrochemical
facies between groundwater and surface water are visible in
52
the high concentration of Mg2+ in the Aniene River. These
hydrochemical facies highlight that carbonate weathering
processes (e.g. calcite and dolomite) are the most important
factors of the observed water type.
Thus, the scatter plot of (Ca2++Mg2+) versus (SO42-+HCO3-)
was prepared to identify the ionic exchange and weathering
processes (Fig.4). As shown in Fig.4, all surface water samples
are close to the 1:1 equiline suggesting that those ions have
resulted from weathering of carbonates. However, water
samples from Pertuso Spring are clustered under the 1:1 line
indicating ion exchange.
To study the difference in Mg2+ content between
groundwater and surface water, a ternary diagram of cations
(Ca2+, Mg2+ and Na++K+) (Edmunds et al. 1987) was used
to highlight how the weathering type processes influence the
enrichment in Magnesium (Fig.5).
Fig. 5 - Ternary diagram of cations Ca2+, Mg2+ and Na++K+. Relative concentrations
of dissolved major cations compared with the composition of local groundwater.
Fig. 5 - Diagramma ternario dei cationi Ca2+, Mg2+ and Na++K+. Concentrazioni relative dei principali cationi disciolti in confronto alla composizione delle
acque sotterranee locali.
Fig. 4 - Scatter diagram of (Ca2++Mg2+) versus (SO42-+HCO3-).
Fig. 4 - Diagramma a dispersione di (Ca2++Mg2+) versus (SO42-+HCO3-).
As shown in Fig.5 all samples are placed along the Ca2+Mg2+ side of the diagram, these groundwater samples are
very poor in Na+ and K+. The higher Mg2+ concentrations
in SW_01 water samples suggest an increase in residence
time and depend on the dissolution/precipitation reactions of
calcite and dolomite, which occur in the aquifer.
Based on previous studies [ACEA ATO 2 S.p.A.,
unpublished data (2005)] similar results have been obtained
for other springs located in the upper part of Aniene River
coming out in the Triassic dolomite outcropping close to the
Pertuso Spring basin (Fig.6).
This hydrochemical characterization leads to a groundwater
flow pattern in which two main recharge areas are defined:
the first one in the Cretaceous limestone, feeding the Pertuso
Spring groundwater; the second one in the Triassic dolomite,
feeding the surface water upstream the spring. Saturation
indexes calculated with respect to calcite and dolomite of
Pertuso Spring and surface water are shown in Figs. 7 and 8.
All surface water samples are saturated with respect to calcite
(Fig. 7) and dolomite (Fig. 8), while all Pertuso Spring samples
are saturated with respect to calcite and undersaturated
with respect to dolomite. The saturation with respect to
calcite and dolomite reflects a great dissolution and strong
mineralization along groundwater flowpaths. The scatter
plots diagram for Mg2+/Ca2+ ratio (Fig. 9) shows that the
Fig. 6 - Recharge areas of the main karst springs in the Upper Valley of
Aniene river.
Fig. 6 - Aree di ricarica delle principali sorgenti carsiche nell’Alta Valle
dell’Aniene.
53
increase in Mg2+ concentrations in surface water, upstream
the Pertuso Spring, and consequently Mg2+/Ca2+ ratio may be
due to the weathering of Mg-rich Triassic dolomites, where
dolomitic limestones and dolomites are the most outcropping
formations in this area.
As regards Pertuso Spring groundwater, the high Mg2+/
Ca2+ ratios (~0.5) mainly depend on the residence of water
in the karst system, highlighting long residence time and
enhanced weathering along the groundwater flowpaths of
low-Mg calcite. As a consequence of these properties, water
samples taken downstream Pertuso Spring (SW_02) present
chemical composition typical of mixing of these two different
kinds of waters (Fig. 9).
As a matter of fact, the highest Mg2+/Ca2+ ratio has been
recorded in water samples coming from dolomite rock masses
(SW_01~0.7), with medium value at SW_02 gauging station
(~0.4). On the other hand, the lowest value of this ratio in
groundwater has been observed in Cretaceous limestone area
(~0.3) (Tab.3).
Aniene River discharge was measured during the
hydrological year 2014-2015, in order to cover the range
of seasonal conditions characteristics of this complex
hydrogeological system. Measurements by current meter were
carried on in two gauging stations located upstream (SW_01)
and downstream (SW_02) Pertuso Spring (Tab.4).
Tab.5 shows discharge values recorded in SW_01 and
SW_02 gauging stations all over the hydrological year. It can
be noticed that the average discharge value referred to SW_01
gauging station can be easily compared with the total average
discharge coming from the most important karst springs
outcropping in the Upper Part of Aniene River (Tab.1).
Thus, the source of Mg2+ concentration values in Aniene
River upstream Pertuso Spring (SW_01) is the dissolution of
Magnesium rich minerals in Triassic dolomites, sited in the
north-east part of the Pertuso Spring basin. Along the Aniene
River, this decrease in Mg2+ concentration values is related to
an increase in stream flow discharge. SW_02 surface water is
the product of the confluence of groundwater coming from
Pertuso Spring into the Aniene River (SW_01). As a matter
of fact, Aniene River water, which is characterized by water
with higher Magnesium concentration values, is affected in
its chemical composition by Pertuso Spring groundwater
inflowing, and this influence can be measured by Mg2+
concentration values variability along the river downstream.
The karst aquifer system has been studied in order to
evaluate factors which modify the Aniene River flow due to
groundwater-surface water interactions.
The main inputs are the discharge measurements (Q1) and
the Mg2+ concentrations (C1) recorded upstream the spring
(SW_01). The secondary inputs are the discharge rate (QP)
and the Mg2+ concentrations (CP) recorded at Pertuso Spring.
The main output is the Aniene River discharge (Q2) and the
Mg2+ concentrations (C2) recorded at the SW_02 gauging
station located downstream Pertuso Spring.
The area under study belongs to a special protected area
in the Natural Park of Simbruini Mountains, where all the
54
Fig. 7 - Saturation indexes of Calcite of groundwater and surface water.
Fig. 7 - Indice di saturazione in calcite delle acque sotterranee e superficiali.
Fig. 8 - Saturation indexes of Dolomite of groundwater and surface water.
Fig. 8 - Indice di saturazione in dolomite delle acque sotterranee e superficiali.
Fig. 9 - Mg2+/Ca2+ ratio versus HCO3- in groundwater and surface water.
Fig. 9 - Mg2+/Ca2+ ratio versus HCO3- nelle acque sotterranee e superficiali.
Tab. 3 - Mg2+/Ca2+ ratio from Pertuso Spring and Aniene River gauging station.
Tab. 3 - Rapporto Mg2+/Ca2+ della sorgente Pertuso e delle sezioni di misura
del Fiume Aniene.
Mg2+/Ca2+
Pertuso Spring
SW_01
SW_02
July 2014
0.328
0.728
0.410
November 2014
0.267
0.669
0.357
January 2015
0.305
0.718
0.386
May 2015
0.314
0.752
0.391
December 2015
0.332
0.741
0.458
Tab. 4 - Mean discharge values obtain by current meter method, upstream (SW_01)
Thus, the discharge rate of Pertuso Spring depends on the
discharge values measured in the gauging station located
along the river upstream the spring (Q1) and the Mg2+
concentration values recorded in groundwater and surface
water samples (C1, C2, CP) (5):
and downstream (SW_02) Pertuso Spring.
Tab. 4 - Valori della portata media ottenuti con il metodo correntometrico, a
monte (SW_01) e a valle (SW_02) della sorgente Pertuso.
Q (m3/s)
SW_01
SW_02
July 2014
0.540
2.450
November 2014
0.350
1.480
January 2015
0.410
1.920
May 2015
0.501
2.747
December 2015
0.278
0.931
Q=
Q1 ⋅
P
(2)
Applying the conservation of mass equation to this closed
system, it means (3):
Q1C1 + QP CP =
Q2C2
(3)
The n parameter (Tab.5), i.e. the percentage of Pertuso
Spring groundwater contribution to total discharge measured
at SW_02 is defined according to (4).
=
n
QP ( C2 − C1 )
=
Q2 (CP − C1 )
(5)
The Mg2+ versus Q scatter plots diagram (Fig. 10) shows
that all plotted points, except for November 2014 data, follow
a linear trend, highlighting that the increasing contribution
of Pertuso Spring flow rate is responsible of the decreasing
values of Mg2+ concentration. Data, referred to November
2014 monitoring campaign, have to be considered as an
outlier (Fig. 10). This lower Mg2+ concentration measured in
November 2014 can be related to a high precipitation rate,
recorded at Trevi nel Lazio meteorological station, during five
days before the sampling date, which could have influenced,
by dilution, the Mg2+ concentration in groundwater.
Pertuso Spring discharge values obtained by this indirect
method are showed in Tab. 6.
The relationship between Mg2+ concentration and karst
spring discharge values obtained with this combined
Magnesium-discharge tracer approach is represented in Fig. 10.
anthropic activities are restricted and controlled. On the
basis of this hypothesis we have assumed that this system
could be considered closed. Thus, the SW_02 gauging station
discharge values, come from the contribution of Pertuso
Spring discharge (QP) to the original SW_01 discharge value,
can be represented by (2):
Q=
Q1 + QP
2
n
1− n
(4)
Tab. 5 - Values as percentage contribution of Pertuso Spring groundwater to total discharge measured at the SW_02 gauging station.
Tab. 5 - Valori del parametro n come contributo percentuale fornito dalle acque sotterranee della sorgente Pertuso alla portata complessiva misurata nella
sezione di misura SW_02.
Date
n
n (%)
July 2014
0.780
78.0
November 2014
0.752
75.2
January 2015
0.794
79.4
Fig. 10 - Correlazione tra la concentrazione in Mg2+ e i valori di portata della
May 2015
0.812
81.2
sorgente Pertuso ottenuti con l’approccio combinato magnesio-portata.
December 2015
0.709
70.9
Fig. 10 - Relationship between the Mg2+concentration and Pertuso Spring discharge
values obtained with the combined Magnesium-discharge tracer approach.
Tab. 6 - Magnesium content and discharge values obtained by current-meter and Magnesium tracer method (Q*: discharge values obtained by current meter method; Q**: discharge values obtained by the difference between the values measured with the current meter in SW_01 and SW_02).
Tab. 6 - Tenore di magnesio e valori di portata attenuti con il metodo correntometrico (Q*: valori di portata attenuti con il metodo correntometrico; Q**: valori
di portata attenuti come differenza tra I valori misurati con il metodo correntometrico nelle sezioni SW_01 e SW_02).
Date
July 2014
SW_01
Q*
Pertuso Spring
(m3/s)
Mg2+ (meq/L)
0.54
1.94
1.91
Q**
(m3/s)
SW_02
Mg2+ (meq/L)
Q* (m3/s)
QMg (m3/s)
1.92
0.80
2.45
2.46
1.05
QMg
(m3/s)
Mg2+ (meq/L)
November 2014
0.35
1.93
1.13
1.06
0.68
1.48
1.41
0.99
January 2015
0.41
2.07
1.51
1.58
0.81
1.92
1.99
1.07
May 2015
0.50
2.02
2.23
2.16
0.77
2.75
2.66
1.00
December 2015
0.28
2.07
0.67
0.68
0.86
0.93
0.96
1.21
55
Conclusions
This paper dealt with the assessment of the interactions
between karst aquifers feeding Pertuso Spring and Aniene
River surface waters on the basis of stream discharge
measurements and water geochemical tracers data. The aim
was to set up an inverse model, which allowed estimating
groundwater flow coming out from Pertuso Spring, starting
from surface water discharge measurements and geochemical
waters characterization. These preliminary results, obtained
in area under study, sited in the karst aquifer outcropping in
the Upper Valley of Aniene River, show that it is possible to
have a reliable evaluation of Pertuso Spring discharge, through
the elaboration of surface water discharge measurements and
Mg2+ concentration values, determined both for groundwater,
coming from Pertuso Spring, both for surface water sample,
collected upstream and downstream Pertuso Spring, along the
Aniene River. Although it is subject to some uncertainties,
the Mg2+ concentration, as an environmental tracer, provides
an indirect method for discharge evaluation of Pertuso Spring
due to the mixing of surface water and groundwater, and
it provides information o n changes in water quantity and
quality.
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